import pprint
with open('264.txt') as file_obj_1, open('247.txt') as file_obj_2:
    domain_264 = set([_.replace('\n', '').lower() for _ in file_obj_1])
    domain_247 = set([_.replace('\n', '').lower() for _ in file_obj_2])
    pprint.pprint(domain_264 - domain_247)